Before retreating into their Soyuz, they embraced the three colleagues they were leaving behind at the 400-kilometre-high complex.

The station's newest commander, Randy Bresnik, noted the outpost was losing 1,474 days of spaceflight experience with the departure of Dr Whitson, Dr Yurchikhin and Mr Fischer — four years and two weeks, he pointed out.

"We are in your debt for the supreme dedication that you guys have to the human mission of exploration," Mr Bresnik told them on the eve of their departure.

He offered up special praise for Dr Whitson — "American space ninja" — and wished them all Godspeed.

Whitson looking forward to pizza and flushing toilets

Dr Whitson, a biochemist, set a breakneck pace on all three of her space station expeditions, continually asking for more — and still more — scientific research to do, and scientists on the ground said it often was hard to keep up with her.

She even experimented on food up there, trying to add some pizazz to the standard freeze-dried meals — tortillas transformed into apple pies on her watch.

Dr Whitson was supposed to fly back in June after six months in space but when an extra seat opened up on this Soyuz, she jumped at the chance to stay in orbit an extra three months.

Only one other American — yearlong spaceman Scott Kelly — has spent longer in space on a single mission.